Quantification of virtual chemical properties
Wu, Judy I-Chia
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Highly important chemical concepts, like strain, hyperconjugation, conjugation, and aromaticity, are widely used to interpret the behavior of organic molecules, but are virtual (i.e., not directly measurable). This dissertation focuses on validating reliable quantum mechanical approaches for quantitative estimates of these virtual chemical properties and applying them to solve significant chemical problems. We are concerned with the simplest iconic organic molecules: cyclopropane, benzene, cyclobutadiene, cyclooctatetraene, and the polybenzenoid hydrocarbons. Cyclopropane’s unexpectedly low ring strain (almost the same as that of cyclobutane) is due to substantial CCC geminal delocalization. Cyclooctatetraene (COT) is not representative of an unconjugated cyclic polyene. Instead, double hyperconjugation stabilizes the tub-shaped COT (D2d), as well as other non-planar conjugated systems, and compensates for their diminished π conjugation. On the other hand, planar D4h COT is net stabilized by π conjugation and only weakly anti-aromatic destabilized. Planar annulenes in general are not very destabilized antiaromatically. Even cyclobutadiene is only modestly destabilized by antiaromaticity; its high heat of formation is mainly due to significant angle strain and Pauli repulsion between the pairs of C=C π bonds. Large 4n π electron polycyclic benzenoids and the higher dihydrodiazaacenes benefit from substantial π conjugation and can even display aromatic stabilization energies (ASE), of equal or greater magnitude, than their 4n+2 π electron analogs. Aromaticity is very robust towards electronic and geometric perturbtations. Hence, perfluorobenzene (C6F6) is as aromatic as benzene, while perfluorocyclobutadiene (C4F4) has only reduced antiaromaticity compared to cyclobutadiene, due to the twisting of its π system. This work further shows the effectiveness of the nucleus independent chemical shifts (NICS) as a “local” probe of aromaticity for substituted and fused-ring aromatic systems. Our computational findings provide valuable insight to the development of fundamental organic concepts.